1. Technical Field
The disclosure relates to an optical device, and a laser chamber and a gas laser apparatus using the same.
2. Related Art
The miniaturization and enhanced levels of integration of semiconductor integrated circuits have led to a demand for increases in the resolution of semiconductor exposure devices (called “exposure devices” hereinafter). Accordingly, advances are being made in the reduction of the wavelengths of light emitted from exposure light sources. Gas laser devices are being used as exposure light sources instead of conventional mercury lamps. At present, a KrF excimer laser device that emits ultraviolet light at a wavelength of 248 nm and an ArF excimer laser device that emits ultraviolet light at a wavelength of 193 nm are being used as gas laser devices for exposure.
Immersion exposure, in which the apparent wavelength of an exposure light source is reduced by filling the space between the exposure lens of an exposure device and a wafer with a liquid and changing the refractive index, has been practiced as the present exposure technique. In the case where immersion exposure is carried out using an ArF excimer laser device as the exposure light source, the wafer is irradiated with ultraviolet light at a wavelength of 134 nm within the liquid. This technique is referred to as ArF immersion exposure (or ArF immersion lithography).
The natural oscillation amplitude of a KrF or ArF excimer laser device is as wide as 350-400 pm, and thus chromatic aberration will occur at laser light (ultrasonic light) reducedly projected on the wafer if the projection lens in the device is used, leading to a drop in the resolution. It is therefore necessary to narrow the spectral line width of the laser beam emitted from the gas laser device until the chromatic aberration reaches a level that can be ignored. Here, a spectral line width is also called a spectral width. Accordingly, the spectral width has been narrowed by providing a line narrowing module (LNM) having a line narrowing element within the laser resonator of the gas laser device. Here, the line narrowing element may be an etalon, grating or the like. A laser device that narrows the spectral width in this manner is called a narrow-band laser device.